1. Field of the Invention
The present invention relates to a zoom lens system and more particularly to a zoom lens system capable of wide angle photography.
2. Description of the Prior Art
It is well known that a negative lead zoom lens system having a negative front lens group is suitable for a zooming mode of operation in a wide angle photography range since the power distribution of such a zoom lens system is of an inverted telephoto type. The negative lead zoom lens system design, however, is not favorable for increasing the zoom ratio. This is because that the compactness of this type of system along its optical axis is lost if the system is designed with a strained great zoom ratio. Also the difficulty of aberration correction is increased if the zoom ratio is desired to be increased by extending the zoom range toward the telephoto range.
It is also generally known that a conventional positive lead zoom lens system having a positive front lens group is favorable for zooming in a telephoto range and for reducing the bulkiness of the system along the optical axis. In the positive lead zoom lens systems, the focusing operation is preferably accomplished by shifting the positive front lens group. The alternative shifting of the entire lens system for a focusing operation requires a complex barrel mechanism to provide a change in the amount of shift in accordance with focal length variation.
The adoption of a front lens group focusing in the positive lead zoom lens group is limited to photograph of a field angle less than about 60 degrees. If a positive lead zoom lens system with front lens group focusing is desired to be designed so as to enable a wider angle of photograph than a field angle of 60 degrees, the diameter of the front lens group would be greatly increased to permit the light rays in the entire field angle to enter the lens system. This would result in a zoom lens system having a front diameter beyond a practical size. Thus, generally a positive lead zoom lens system is considered unfavorable for wide angle photography. Various zoom lens systems of negative first lens group or positive first lens groups are known in the patented prior art, such as U.S. Pat. No. 3,848,969; U.S. Pat. No. 3,788,731; U.S. Pat. No. 3,771,853; U.S. Pat. No. 3,549,235; U.S. Pat. No. 4,063,800; U.S. Pat. No. Re 29,237; U.S. Pat. No. 3,975,089; U.S. Pat. No. 4,017,161 and U.S. Pat. No. 3,937,562.
There is still a demand for a compact zoom lens capable of wide angle photography with an extended zoom ratio.